Radioreceptor and radioimmunoassays will be used to determine the opiate-like peptide content of the bovine splenic nerve which is comprised of approximately 98% sympathetic C-fibers. In addition, a highly purified fraction of large dense-cored noradrenergic vesicles, which measure approximately 750 Angstrom units average diameter and equilibrate at a density of 1.18 g/cc, will be prepared by the sucrose -D2O density gradient method from this nerve. The opiate-like peptide distribution will be determined at the various steps in the vesicle purification procedure and compared with the distributions of norepinephrine, dopamine, dopamine beta-hydroxylase and ATP. An attempt will be made to determine if the opiate-like peptdes are stored together with norepinephrine, dopamine beta-hydroxylase and ATP in the major large dense-cored vesicle population or whether a minor population of other vesicles may exist with similar density and slightly larger diameter, which co-purify with the large dense-cored vesicles by the procedure employed. Such a minor population of vesicles might be anticipated from the infreqent SIF cells containing 1200 Angstrom units diameter dense-cored vesicles, in which case dopamine rather than norepinephrine is believed to be present and would be co-stored with opiate-like peptides. Data based on identically purified vesicle fractions from equal weights of sequential segments of the bovine splenic nerve will be used to suggest if the opiate-like peptides are pre-packaged into the vesicles in the cell body prior to axoplasmic transport or whether the peptides are synthesized or otherwise accumulated by the vesicles from some endogenous source in the neuroplasm during migration to the terminals.